Non-composited, fixed position should trigger main thread scrolling

In a recent change, I broke the logic for determining whether fixed position
elements should trigger main thread scrolling. This CL fixes the logic to
trigger main thread scrolling if we have fixed position elements that lack
composited layers.

R=vollick@chromium.org
BUG=402359

Review URL: https://codereview.chromium.org/465333004

git-svn-id: svn://svn.chromium.org/blink/trunk@180226 bbb929c8-8fbe-4397-9dbb-9b2b20218538